Initial scaffold: Sinatra map app with Tauri desktop shell

Sinatra backend serving a basic Leaflet map, wrapped in a Tauri
desktop shell that starts the same server in a native window.

public/js/map.js

@@ -0,0 +1,6 @@
+const map = L.map("map").setView([48.1486, 17.1077], 13);
+
+L.tileLayer("https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png", {
+  maxZoom: 19,
+  attribution: "© OpenStreetMap contributors",
+}).addTo(map);